ARMs business is IP so they care about GPLv3 and the constraints it puts them under.
Alternative open-source compilers such as LVVM, with less onerous licensing, are therefore more likely to be contributed to,
ARM will invest very little in GCC now, because of GPLv3.
The question should be is why the benchmark used a generic compiler for ARM (gcc) versus a vendor specific compiler for Intel (icc). Why was the ARM produced compiler not used?